But they all run slightly differently, whether a different screen size, keyboard configuration, or flavor of the Android operating system. This is bad for Google.

Here's one practical example of why that's such a bad thing: Twitter's new official Android app, announced today, only works on Android phones running 2.1 or newer versions of the Android operating system.

That includes the Nexus One and the new HTC Droid Incredible, but doesn't include a bunch of other Android phones, including some new ones on the market. (And users generally don't have any control over this -- it's up to handset makers and operators.)

Good luck to Twitter's support folks who will have to explain to a bunch of confused Android users about why their phone can't use the new Twitter app. (Though this leaves a door open for other developers who make Twitter apps for older versions of Android. And, of course, Twitter will still work for those users on the mobile web and via SMS.)

Mostly, it's indicative of how much of a nuisance and competitive disadvantage the fragmented Android market could become if this problem continues.

Fragmentation isn't as much of a problem for Apple's iPhone, but it could become more of one as Apple continues to update its operating system with new features that don't work on its older phones. But today, Apple remains far less fragmented than Android.